Omar Mateen, who opened fire at Pulse nightclub killing at least 49 people on Sunday, called News 13 and spoke to producer Matthew Gentili (pictured) mid-rampage

Mateen (left) told Gentili (right) that he was carrying out the atrocity in the name of terrorist group ISIS

Mateen then announced that he was carrying out the atrocity in the name of ISIS.

'He did it for ISIS, and he started speaking Arabic,' Gentili said. 'At the time, I didn't know what he was saying. He was speaking so fast. But it was ... he was speaking fluently.'

The shooter, an American of Afghan descent who the FBI believes was radicalized online, burst into the Pulse nightclub in Orlando early Sunday armed with a .223 caliber AR-15 assault rifle and a handgun.

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By the time his shooting rampage ended, 49 people were dead and 53 others injured.

Mateen, who was probed in recent years for suspected extremism, is believed to have made the phone call to News 13 while he was holed up in the bathroom of Pulse nightclub - with his effective hostages.

It followed a call he had made to 911 where the terrorist had once again pledged his allegiance to ISIS.

He had also called a friend to say goodbye during the attack, according to law enforcement officials.

The FBI are speaking to the recipient but have not released any further details about what was said during the phone call.

The shooter is believed to have made the phone call to News 13 while he was hauled up in the bathroom of Pulse nightclub (pictured on Sunday after the attack)

He died in a gun battle with police a short time later.

Gentili said that his conversation with the Islamic extremist will haunt him.

'I will never forget the words he said to me,' he said.

The TV producer said he had answered the phone to hear the caller ask: 'Do you know about the shooting?'

After he confirmed that he did, Mateen told him that he was the shooter and that he was carrying out the attack 'for ISIS' before lapsing into Arabic.

After imploring the caller to speak in English, Mateen once again pledged allegiance to the terrorist group.

'I did it for ISIS,' he said. 'I did it for the Islamic State.'

Gentili said he asked where the caller was, who told him it was 'none of my f***ing business.'

Finally, the producer asked if there was anything else Mateen wanted to say.

'He said no and hung up the phone,' he said.

News 13 confirmed that the phone number the caller rang from belonged to one registered to Omar Mateen.